Dual-tone multi-frequency signaling

Dual-tone multi-frequency signaling (DTMF) is an in-bandtelecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized by ITU-T Recommendation Q.23. It is also known in the UK as MF4.

The Touch-Tone system using a telephone keypad gradually replaced the use of rotary dial and has become the industry standard for landline and mobile service. Other multi-frequency systems are used for internal signaling within the telephone network.

Multifrequency signaling

Prior to the development of DTMF, telephone numbers were dialed by users with a loop-disconnect (LD) signaling, more commonly known as pulse dialing (dial pulse, DP) in the U.S. It functions by interrupting the current in the local loop between the telephone exchange and the calling party's telephone at a precise rate with a switch in the telephone that is operated by the rotary dial as it spins back to its rest position after having been rotated to each desired number. The exchange equipment responds to the dial pulses either directly by operating relays, or by storing the number in a digit register recording the dialed number. The physical distance for which this type of dialing was possible was restricted by electrical distortions and was only possible on direct metallic links between end points of a line. Placing calls over longer distances required either operator assistance or provision of special subscriber trunk dialing equipment. Operators used an earlier type of multi-frequency signaling.

TouchTone

TouchTone is a 2015 puzzle video game for iOS devices by Mikengreg, a two-person indie game development team made up of Mike Boxleiter and Greg Wohlwend. The player monitors phone calls as part of a government surveillance program to find public threats. The player unlocks chains of emails by completing a series of puzzles wherein a beam is reflected around a room to a set destination. TouchTone's core concept grew from a two-day game jam immediately following their 2012 release of Gasketball, but only found its hacker theme following the mid-2013 Edward Snowdenglobal surveillance disclosures. The tone of TouchTone's story grew from satirical to serious over the course of the game's development.

The game was released on March 19, 2015. Review aggregatorMetacritic characterized TouchTone's reviews as generally favorable. Critics praised the game's visual style and story, but criticized the way the game did not allow players to skip puzzles. Reviewers found the light-bending puzzle premise unoriginal, but appreciated its thematic connection.

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